The metabolism of genistein, a phytoestrogen derived from varius soy products, was investigated with rat and human liver microsomes and various recombinant human cytochrome P450 enzymes. The microsomes obtained from rats treated with pyridine, phenobarbital, (-naphthoflavone, pregnenolone-16(-carbonitrile, or 3-methylcholanthrene resulted in very different product profiles consisting of five different metabolites as observed by HPLC reversed-phase analysis at 260 nm. The metabolism of genistein was also investigated with recombinant human cytochrome P450 1A1, 1A2, 1B1, 2B6, 2C8, 2E1, or 3A4. P450s 1A1, 1A2, and 1B1 metabolized genistein to predominantly form one product (peak 3) with lesser amounts of peaks 1 and 2. P450 3A4 produced high levels of peaks 1,2 and 3. These three products eluted off the HPLC column prior to the starting material genistein and the UV-VIS spectrum and GC-MS analysis of peak 3 support that this product results from hydroxylation of ge nistein.